At present, an RH or DH method is generally adopted for degassing or adjustment of molten steel composition. FIG. 1 shows an embodiment of the RH method. A treating method by use of an apparatus as shown in FIG. 1 is explained hereinafter.
Denoted as numerals 1, 2, 3, 4, 5, 6 and 7 are a ladle, a mixing vessel, a molten steel ascending pipe, a molten steel descending pipe, an inert gas (in general, Ar) feeding pipe, a vacuum suction port and an alloy element containing vessel, respectively. Denoted as A and B are molten steel and alloy elements. When a method by use of the apparatus as shown in FIG. 1 is performed, the mixing vessel 2 is disposed over the ladle 1, both the ascending pipe 3 and the descending pipe 4 are dipped in molten steel A in the ladle 1 and the mixing vessel is sucked from the vacuum suction port 6 to obtain a vacuum condition. The pressure difference between the ladle 1 and the mixing vessel 2 becomes approximately 1 atm. and molten steel A is sucked up to the mixing vessel 2. (When the pressure difference is 1 atm., the suction height is approximately 1.48 m.) An inert gas such as Ar is fed to the molten steel ascending pipe 3 through the inert gas feeding pipe 5 to thereby lift up molten steel A to the mixing vessel 2 in accordance with the theory of an air lift pump.
When molten steel A reaches the point of the aforesaid suction height, excess molten steel A flows down to the ladle 1 through the descending pipe 4. In this manner, molten steel A is circulated between the ladle 1 and the mixing vessel 2, and gases such as O.sub.2, H.sub.2, N.sub.2 and the like which dissolve in molten steel A are removed while molten steel A stays in the mixing vessel 2 as the mixing vessel 2 is maintained in a vacuum condition during this time. Alloy elements B are added to molten steel A in the mixing vessel 2 from the alloy element containing vessel 7. Thereafter, alloy elements B are mixed with molten steel A in the mixing vessel 2 then in the ladle 1, in which alloy elements B are uniformly dispersed into molten steel A by circulating molten steel A for a predetermined period of time.
However, this method causes some problems mentioned below due to the feeding of an inert gas, which are regarded as the principal difficulties for the practice of this method. An inert gas to be introduced to the ascending pipe 3 through the inert gas feeding pipe 5 appears over the surface of molten steel A in the mixing vessel 2 and is then exhausted toward the vacuum suction port 6. A part of alloy powder added from the alloy element containing vessel 7 also flows toward the vacuum suction port 6 together with the inert gas. Such an alloy powder adheres to an inner wall of the mixing vessel 2 or flows out of the mixing vessel 2 along the suction line. This results in the loss of a substantial amount of added alloy elements, so that an expected adjustment of molten steel composition can not be achieved. Further, alloy powder adheres to or accumulates in the suction line, which causes various problems such as a detrimental influence on the vacuum pump, from the view point of maintenance of attached apparatuses.
Furthermore, in the RH or DH method, the temperature of molten steel A gradually decreases while being circulated, and a dissolution rate of alloy elements also decreases extremely due to decrease of the temperature. Therefore, the time necessary for the adjustment of molten steel composition (time necessary for dissolving alloy elements in molten steel uniformly) can not be reduced even if the time necessary for circulation of molten steel is shortened.
There is an another method for lifting up molten steel from the ladle to the mixing vessel which utilizes a electromagnetic pump. This method adopts a power source having a low frequency of 0.9 to 16 cycles, therefore such is not expected to avoid the decrease of temperature of molten steel because such a low frequency does not produce an effect of heating molten steel as well as the RH or DH method. Further, specific consideration is not given to a descending flow of molten steel in this method, so that the efficiency of dissolution of alloy elements or degassing is still not improved and it will therefore take a great deal of time to complete an expected dissolution of alloy elements or degassing. Furthermore, as this method adopts the aforesaid specific low frequency, cost for electric power source becomes two or three times greater than that of an apparatus utilizing a commercial frequency. In summary, it will be difficult to perform this method on an industrial scale.